1. Field of the Invention
The present invention relates to a waveguide type optical device, and particularly, to an optical device using 3rd order nonlinear optical material.
2. Description of the Background Art
In order to implement an all-optical device such as optical switches and optical modulators for data processing at higher speed, nonlinear refractive index change, one of 3rd order nonlinear optical effects, can be used. In the phenomenon, refractive index of a medium is changed due to change of incident light intensity.
The 3rd order nonlinear optical effect in a medium can be classified into two kinds depending on its generation mechanism. In one of them, 3rd order nonlinear optical effect is achieved in a state that laser beam incident on an optical material is little absorbed. In this type, not resonance such as excitation of a material due to light absorption, but ‘non-resonance effect’, such as optical Kerr effect in a silica glass, is used. This type of 3rd order nonlinear optical effect has advantages such as less light absorption, fast response time, but it also has disadvantages of very small 3rd order nonlinear optical coefficient value.
To overcome such a disadvantage, some devices using a silica optical fiber as a waveguide have been proposed, so that incident light reacts in a long distance in a medium. However, such devices are impossible to be integrated because of its long waveguide. Also, the devices are very sensitive to temperature change, which deteriorates device performance, and thus have poor industrial utility.
In the other type of 3rd order nonlinear optical effect, incident light of a vicinity of an absorption edge of an optical material or incident light having a wavelength close to excited resonance wavelength of a semiconductor is made to react to a nonlinear material, so that a nonlinear refractive index is induced by a resonance phenomenon. Because a very large 3rd order nonlinear optical effect can be induced even with small intensity of light in this type, a large refractive index change can be generated even with a low power. However, this type is disadvantageous in that light cannot be guided in a long distance because linear absorption of light in a material is great, and that its response time is slower compared to the non-resonance type.
Of materials for generating a 3rd order nonlinear optical phenomenon by resonance, a composite film where metal particles or semiconductor particles are dispersed in a transparent dielectric, a semiconductor or a polymer matrix is known for its large 3rd order nonlinear optical susceptibility (X(3)) and its response time similar to that of the non-resonance effect. Composite material obtained by dispersing metal or semiconductor particles of nanometer size in a transparent dielectric matrix such as a silica or the like is advantageous in that it shows a very large 3rd order nonlinear optical susceptibility value via a dielectric confinement effect and a quantum confinement effect, and its linear and nonlinear optical properties can be easily tailored through various combinations between constituents.
In a composite material with semiconductor nano-particle dispersion, the quantum confinement effect is dominant mechanism for 3rd order nonlinear optical phenomenon. This composite material is disadvantageous in that an operation speed is limited to tens of pico seconds at best because it requires interband transition of a real charge carrier, but is advantageous in that a 3rd order nonlinear susceptibility is large and its operation wavelength (band) can be tailored within a relatively large wavelength range only by changing its particle size.
Meanwhile, a composite material with metal nano-particle dispersion utilizes 3rd order nonlinear optical phenomenon caused by a dielectric confinement effect in which local electric field around the metal particles is enhanced by a surface plasmon resonance. This type of composite material has somewhat restrictive in changing operation wavelength compared to the composite materials made by dispersing semiconductor particles. This composite material is advantageous in that its 3rd order nonlinear optical susceptibility value is similar to that of the semiconductor-dispersed composite material, and that it has a response time of pico seconds or less due to an instant-reaction characteristic of an optical Kerr effect, and that it is chemically stable as well as stubborn to laser irradiation.
Accordingly, a very large 3rd order nonlinear optical phenomenon is generated by using such materials, thereby making it possible to give body to an integrated optical device operable with low power. Also, because of its fast response time, the composite materials are promising candidate material for various signal processing devices for high speed optical communication of next generation such as optical switch, optical modulator and logic devices for optical computing.
However, 3rd order nonlinear optical materials made of nano-composite using semiconductor nano-particles absorb light of the corresponding wavelength for interband transition of electrons. Also, the nano-composite material using metal nano-particles disadvantageously has large absorption of light in the wavelength region of surface plasmon resonance in which 3rd order nonlinear optical effect is maximized. The absorption makes actual embodiment of a waveguide type optical device very difficult.
FIG. 1 is a graph showing an absorption coefficient change as a function of wavelength for a nano-composite material (Au—SiO2) obtained by dispersing gold (Au) nano-particles in a silica matrix as one example of an optical material generating a large 3rd order nonlinear optical phenomenon.
As shown in FIG. 1, the gold-silica (Au—SiO2) nano-composite thin film has large absorption coefficient in a wavelength shorter than 700 nanometer, but has very small absorption coefficient in an infrared region above 700 nanometer. A peak of the absorption coefficient, shown in a wavelength of about 500 nanometer is generated due to surface plasmon resonance phenomenon. Namely, it means that if a laser beam with wavelength of about 500 nanometer is used, this material can show a large 3rd order nonlinear optical effect. Such composite material absorbs little light in a near infrared region, in which the band of communication wavelength generally used is located. However, because the band of communication wavelength is apart from the resonance wavelength of the material, a refractive index change due to a 3rd order nonlinear optical effect is not large.
Because the gold-silica composite material has a very large 3rd order nonlinear optical coefficient compared to that of silica based material commonly used as an optical fiber, it can be used for manufacturing a waveguide type optical device.
Typical example of such an optical device is a Mach-Zehnder interferometer optical switch shown in FIG. 2. In this switch, since the material shows large absorption of a guided light with wavelength at which the material has large 3rd order nonlinearity, a waveguide made of such nano-composite material as a whole cannot guide a light with such wavelength.
Theoretically, as shown in FIG. 2, it is possible to insert a material with large 3rd order nonlinearity 3 into only a part of the waveguide. Through following mechanism, a refractive index is possibly changed by only an incident beam. The incident beam 5 is entered at an input port 2a and is split into two beams at a junction point 2c, and the two beams respectively direct through different arms. When the beams meet with each other at another junction point 2d, if their phases coincide with each other, a constructive interference occurs, thereby outputting a signal beam from an output port 2f. On the contrary, if the phases of two beams are opposite at the moment of the interference, thereby causing a destructive interference, no light is outputted from the output port 2f. 
The length of a waveguide and the distribution of refractive index in Mach-Zehnder are designed such that if a beam passing through an arm including a nonlinear optical material part 3 has sufficiently low intensity, no change in refractive index in the nonlinear optical material part 3 is induced, resulting in a constructive interference at the junction point 2d where two beams meet with each other, thereby outputting an signal beam 5a. In contrast, if the length of the waveguide and the distribution of refractive index (distribution) are designed such that the refractive index is changed by a light with intensity large enough to cause a 3rd order nonlinear optical phenomenon while the beam passes through the part 3, thus a destructive interference occurs at the point 2d, thereby outputting no signal beam 5a. By such a design, an optical switching device can be implemented.
As shown in FIG. 2, a waveguide type optical device may be designed such that a signal beam 5 and a pump beam 6 are separately inserted into the waveguide and a switching operation is made by constructive/destructive interference at an output port 2f of the waveguide depending on whether the pump beam exists or not, while no change in refractive index occurring at the part made of a nonlinear material by an intensity change of incident light. If there is no pump beam 6, a signal beam 5 is emerged from the output port 2f with constructive interference. If there is a pump beam causing a 3rd order nonlinear optical phenomenon, destructive interference of the signal beam occurs at the output port 2f of the waveguide, thereby outputting no signal beam. Such a pump beam can be simultaneously inserted into the input port 2a together with the signal beam, and can be inserted into a separate waveguide 2b. 
Switching operation is theoretically possible in such an optical switching device having the above described structure. However, its manufacturing is practically difficult since the waveguide should include a 3rd order nonlinear optical material part 3 therein. This is because, in order to make no signal beam loss on a waveguide as well as no change in phase of propagating beam by geometrical effect, a refractive index of a waveguide part 2 formed of a general waveguide material such as a silica or polymer with little absorption of both signal beam and pump beam should be matched with that of the waveguide part 3 formed of a 3rd order nonlinear optical material and the two parts 2 and 3 should be connected without interface ununiformity.
However, matching the refractive indices is very difficult. Furthermore, even though the refractive index matching is realized, it is almost impossible to remove interfacial loss in a manufacturing process. Accordingly, it is very difficult to implement the 3rd order nonlinear optical switching device shown in FIG. 2.
Accordingly, the necessity has been proposed for an effective all-optical communication device as follows: overcoming difficulties generated by forming a waveguide in conventional manufacturing processes using two different kinds of materials for one waveguide, and removing refractive index mismatch between the different kinds of materials and a loss of a signal beam due to interface ununiformity.